Mass Mortality Caused by Highly Pathogenic Influenza A(H5N1) Virus in Sandwich Terns, the Netherlands, 2022

Abstract

T he 2021-2022 epidemic of highly pathogenic avian influenza (HPAI) A(H5N1) virus clade 2.3.4.4b has been unprecedented in terms of numbers of dead wild birds, species affected, spatial extent, and incidence in spring 2022 (1). Across Europe, multiple colony-breeding seabirds experienced HPAI H5N1-associated mass mortalities during the breeding period, including the Sandwich tern (Thalasseus sandvicensis) (1,2). The Netherlands constitutes a major though vulnerable stronghold of the Sandwich tern in Europe; 15,000-20,000 breeding pairs have been documented across 10 colonies (https://stats. sovon.nl/stats/soort/6110). We sought to establish the scale of mortality occurring in Sandwich terns breeding in the Netherlands in 2022, characterize the associated HPAI H5N1 viruses and pathology, report on the carcass removal effort relative to survival, and investigate intracolony transmission dynamics.